1. Field of the Invention
This invention relates to detergent tablets, more especially for dishwashing machines, and to their use in the prerinse and main wash cycles of automatic dishwashing machines.
Dishwashing in dishwashing machines generally comprises a prerinse cycle, a main-wash cycle, one or more intermediate rinse cycles, a clear-rinse cycle and a drying cycle. This applies both to domestic and to institutional dishwashing.
Hitherto, it has been standard practice in domestic dishwashing machines, hereinafter referred to as DDWM, to store the detergent in a dispensing compartment which is generally situated in the door of the machine and which opens automatically at the beginning of the main-wash cycle. The previous prerinse cycle is completed solely with cold tapwater flowing into the machine.
In institutional dishwashing machines, hereinafter referred to as IDWM, the preliminary clearing zone corresponds in principle to the prerinse cycle of a DDWM. In machine dishwashing in large kitchens, the detergent fed into the main-wash zone is actually used by overflow in the so-called preliminary clearing zone for the supportive, presoftening removal of food remains adhering to the surfaces to be cleaned. Although there are also IDWM in which the preliminary clearing zone is supplied solely with fresh water, a preliminary clearing zone supplied with detergent solution is more effective more a preliminary clearing zone supplied solely with fresh water.
An object of the present invention is to apply the broad action principle of the preliminary clearing zone of institutional dishwashing machines to domestic dishwashing machines. The addition of detergents to the actual prerinse cycle was originally regarded as one possibility. However, in tests carried out with standard DDWM detergents, it was found that, in addition to the usual dispensing of the detergent through the dispensing compartment in the door, some of the detergent also had to be introduced into the machine itself. However, it is a well-known problem that flow-deficient regions exist both at the bottom of the machine and in the liquor sump of the machine. As a result, the product can never be adequately dissolved and, on completion of the prerinse cycle, has to be pumped off virtually unused.
Scattering detergent into the cutlery basket via the cutlery placed therein is not advisable because irreversible damage can be caused to silver and fine steel.
It has now surprisingly been found that the disadvantages mentioned above do not arise where detergent tablets are used. The introduction of one or more tablets may be effected, for example, in an empty part of the cutlery basket or even elsewhere in the machine.
2. Discussion of Related Art
The use of tablet-form detergents is adequately described in the patent literature. For example, U.S. Pat. No. 3,390,092 describes tablets for dishwashing machines which may be obtained by tabletting a powder-form mixture of sodium silicate having a ratio of Na.sub.2 O to SiO.sub.2 of from 1:3.25 to 2:1 and a water content of from 0 to 20%, alkali metal phosphates, active chlorine compounds, low-foaming nonionic surfactants compatible with the active chlorine compounds, fillers, such as alkali metal carbonates, chlorides or sulfates, white paraffin oil and tablet binders, and which are said to be storable and transportable.
U.S. Pat. No. 4,129,436 describes tablets which essentially contain the same aforementioned constituents but which are said to show particularly high alkalinity which may be achieved inter alia by the addition of alkali metal hydroxide. However, high alkalinity is unsuitable for the domestic use of the detergents because, unless the detergents are properly handled, it can lead to skin irritations and, in addition, can damage decorative finishes.
According to German Patent Application No. 33 15 950, it is particularly advantageous, so far as the required mechanical strength of detergent tablets and their high dissolving rate are concerned, not merely to tablet the mixtures of the consistuents, but instead initially to prepare a co-granulate from the alkaline-reacting constituents and then to tablet the co-granulate thus prepared under high pressure after the addition of further substances and tabletting aids.
In commercial DDWM, all these tablets are introduced into the dispensing compartment also provided for the addition of powder-form or granular detergents which is only designed to open automatically on completion of the prerinse cycle using cold tapwater. After about 5 to 7 minutes, by which time they have been completely flushed out from the dispenser into the dishwashing liquor by the water, the tablets develop their full activity with increasing water temperature during the 20 to 30 minute long main-wash cycle. When the tablets are introduced, for example through the cutlery basket, they enter the prerinse cycle of the machine, but cause increased damage to decorative finishes on account of excessive alkalinity and/or dissolve too quickly and/or disintegrate too quickly and sink without dissolving into the liquor sump of the machine. Therefore, the quantities of detergent available for the main-wash cycle are no longer adequate.